Drive system control method and drive power transmission control system for vehicle

INCORPORATION BY REFERENCE

This application is based on and claims priority under 35 U.S.C. 119 with respect to Japanese Application No. 2003-403963 filed on Dec. 3, 2003, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive system control method and a drive power transmission control system for a vehicle provided with a clutch device, variable in connection force, between a front wheel torque transmission axle and a rear wheel torque transmission axle.

2. Discussion of the Related Art

Heretofore, as described in Japanese unexamined, published patent application No. 7-186766 (186766/1995) for example, there have been known four-wheel drive vehicles of the construction that the connection force of a clutch device provided between a front wheel torque transmission axle and a rear wheel torque transmission axle is kept zero for a predetermined time period subsequence to the starting of the vehicle to make either of front wheels and rear wheels serve as driven wheels and is heightened to bring the vehicle into a four-wheel drive state after the expiration of the predetermined time period.

However, in the four-wheel drive vehicle, either the front wheels or the rear wheels may slip when the four-wheel drive vehicle begins to start, and the connection force of the clutch device between the front wheel torque transmission axle and the rear wheel torque transmission axle is controlled to be heightened for suppression of the slip. This may result in ending the slip to enable the vehicle to start or in causing the both of the front and rear wheels to slip further. In the latter case, some torsion is accumulated on the torque transmission axles while either the front wheels or the rear wheels are slipping but the other wheels are not slipping. When the other wheels slip with the torsion being accumulated on the torque transmission axles, the friction of those wheels with the road surface is changed from a static friction state to a dynamic friction state. This disadvantageously causes the resilient force produced by the torsion to be freed instantaneously, whereby the torque transmission axles are vibrated in the circumferential direction. As a consequence, the driver is given an unpleasant feeling or an anxious feeling.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide an improved drive system control method and an improved drive power transmission control system which are capable of weakening the vibration which is generated by the cause of slip at the time of vehicle starting, to be less than that in the prior art.

Briefly, in a first aspect of the present invention, there is provided a drive system control method for a vehicle wherein a clutch device whose connection force is variable is provided between a front wheel torque transmission axle and a rear wheel torque transmission axle for transmitting a torque of a prime mover to the front wheel torque transmission axle and the rear wheel torque transmission axle for four-wheel traveling of the vehicle and wherein the degree of the connection between the front wheel torque transmission axle and the rear wheel torque transmission axle is variable in dependence on the connection force of the clutch device. The drive system control method comprises a step of judging whether or not one paired wheels of the front wheels and the rear wheels are slipping, in an ordinary mode wherein the vehicle is able to perform the four-wheel traveling; and a step of switching the operation mode of the vehicle from the ordinary mode to an anti-vibration mode in which the connection force of the clutch device is weakened compared to that in the ordinary mode, when the one paired wheels are judged to be slipping.

With this construction, the resilient force produced by the torsion of the torque transmission axles which is caused by the slip of the prime drive wheels or the secondary drive wheels is released by the clutch device provided between the torque transmission axles, so that the vibration which would otherwise be generated when the vehicle performs the four-wheel drive starting can be suppressed compared with that in the prior art.

In a second aspect of the present invention, there is provided a drive power transmission control system for a vehicle comprising a clutch device variable in connection force thereof and provided between a front wheel torque transmission axle and a rear wheel torque transmission axle for transmitting a torque of a prime mover to the front wheel torque transmission axle and the rear wheel torque transmission axle for four-wheel traveling of the vehicle, and a control section for controlling the connection force of the clutch device. The control system further comprises slip state judging means for judging the slip state of either one paired wheels of front wheels and rear wheels when the vehicle is beginning to travel in an ordinary mode for four-wheel drive traveling; slip occurrence judging means responsive to the judgment result of the slip state judging means for judging that the slip has occurred where the slip state continues longer than a predetermined time period or where a slip amount in the slip state exceeds a predetermined amount; and mode switching means responsive to the judgment result of the slip occurrence judging means for switching the operation mode of the vehicle from the ordinary mode to an anti-vibration mode in which the connection force of the clutch device is weakened compared to that in the ordinary mode.

With this construction, where the slip state of the one paired wheels of the front wheels and the rear wheels continues longer than the predetermined time period or where the slip amount in the slip state exceeds the predetermined amount when the vehicle is beginning to travel in the ordinary mode for four-wheel drive traveling, the resilient force produced by the torsion which has been given by the slip on the front and rear wheel torque transmission axles is released by the clutch device provided between the front and rear wheel torque transmission axles, so that the vibration which would otherwise be generated when the vehicle performs the four-wheel drive starting can be suppressed compared with that in the prior art.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The forgoing and other objects and many of the attendant advantages of the present invention may readily be appreciated as the same becomes better understood by reference to the preferred embodiments of the present invention when considered in connection with the accompanying drawings, wherein like reference numerals designate the same or corresponding parts throughout several views, and in which:

<figref idref="DRAWINGS">FIG. 1</figref> is a schematic view of a vehicle in one embodiment according to the present invention;

<figref idref="DRAWINGS">FIG. 2</figref> is a sectional view of a rear differential assembly incorporating a coupling device therein;

<figref idref="DRAWINGS">FIG. 3</figref> is a sectional view of the coupling device;

<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart showing an anti-vibration processing program;

<figref idref="DRAWINGS">FIG. 5</figref> is a flowchart showing the details of the anti-vibration processing program;

<figref idref="DRAWINGS">FIG. 6</figref> is a flowchart showing a phenomenon-dependent anti-vibration processing program; and

<figref idref="DRAWINGS">FIG. 7</figref> is a schematic view showing a vehicle of a so-called “center differential type”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

Hereinafter, a first embodiment of the present invention will be described with reference to <figref idref="DRAWINGS">FIGS. 1 to 5</figref>. <figref idref="DRAWINGS">FIG. 1</figref> shows primary components of a vehicle drive system. In the present embodiment, an engine <b>10</b> as a prime mover is arranged on the front side of a vehicle. Left and right front wheels <b>13</b>, <b>13</b> as prime drive wheels are connected to each other by a front differential <b>11</b>, while left and right rear wheels <b>14</b>, <b>14</b> are connected to each other by a rear differential <b>17</b>.

The front differential <b>11</b> is connected to an output shaft of the engine <b>10</b> through a transmission <b>18</b>. Thus, the output power of the engine <b>10</b> is gear-changed to be transmitted to the front differential <b>11</b>, and the front wheels <b>13</b>, <b>13</b> are driven.

A front end portion of a propeller shaft <b>12</b> is connected to the front differential <b>11</b> through a transfer mechanism (not shown). Further, the propeller shaft <b>12</b> has connected at its rear end portion to one end portion of a coupling device <b>30</b>, which is connected at the other end portion thereof to the rear differential <b>17</b> through another transfer mechanism referred to later.

That is, in the present embodiment, a front wheel torque transmission axle <b>62</b>A is composed of the propeller shaft <b>12</b>, the front differential <b>11</b> and constant velocity joints <b>13</b>A, <b>13</b>A provided between the front differential <b>11</b> and the front wheels <b>13</b>, <b>13</b>, while a rear wheel torque transmission axle <b>62</b>B is composed of the rear differential <b>17</b> and constant velocity joints <b>14</b>A, <b>14</b>A provided between the rear differential <b>17</b> and the rear wheels <b>14</b>, <b>14</b>. A main clutch section <b>39</b> referred to later which is provided in the coupling device <b>30</b> constitutes a clutch device, and the connection force between the front and rear wheel torque transmission axles <b>62</b>A, <b>62</b>B is variable by the main clutch section <b>39</b>.

As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the coupling device <b>30</b>, the rear differential <b>17</b> and the aforementioned transfer mechanism <b>15</b>T are integrated as one unit to constitute a rear differential assembly <b>15</b> for the rear wheels <b>14</b>, <b>14</b>. The transfer mechanism <b>15</b>T is composed of a ring-like transfer bevel gear <b>28</b> which is fixedly fit on the external surface of the rear differential <b>17</b>, and a transfer bevel pinion <b>27</b> which is secured to the rear end portion of the coupling device <b>30</b> in meshing engagement with the ring-like transfer bevel gear <b>28</b>.

As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the coupling device <b>30</b> is provided with a cup-shape outer cylinder case <b>31</b>, and a joint shaft <b>29</b>J extending from a bottom wall of the outer cylinder case <b>31</b> is connected to the propeller shaft <b>12</b>, as schematically shown in <figref idref="DRAWINGS">FIG. 1</figref>. Further, a cap member <b>32</b> is screw-fixed to an opening end of the outer cylinder case <b>31</b>, and a through hole <b>32</b>A formed at the center of the cap member <b>32</b> has an inner shaft <b>33</b> passing therethrough. The inner shaft <b>33</b> is in spline engagement with a transfer bevel pinion shaft <b>27</b>A which is provided with the transfer bevel pinion <b>27</b> at one end thereof, for bodily rotation with the transfer bevel pinion shaft <b>27</b>A.

An axial mid portion of the inner shaft <b>33</b> is formed with a spline shaft portion <b>33</b>S, and plural main inner clutch plates are spline-engaged with the external surface of the spline shaft portion <b>33</b>S. Plural main outer clutch plates are spline-engaged with the internal surface of the other cylinder case <b>31</b>. The main inner clutch plates and the main outer clutch plates are arranged in an alternate fashion to constitute the main clutch section <b>39</b>.

Further, a pilot clutch section <b>35</b> for varying the connection state of the main clutch section <b>39</b> is provided at a position close to the cap member <b>32</b>. The pilot clutch section <b>35</b> serves to axially pressure the main clutch section <b>39</b> in dependence on a magnetic force generated by an electromagnetic coil <b>36</b> which is arranged axially outside of the cap member <b>32</b>. Thus, the connection state of the main clutch section <b>39</b> is changeable to a direct connection state in which the main inner and outer clutch plates of the main clutch section <b>39</b> are engaged to be bodily, a separation state in which the main inner and outer clutch plates of the main clutch section <b>39</b> are separated from one another, and a half clutch state which is between the direct connection state and the separation state.

The electromagnetic coil <b>36</b> of the coupling device <b>30</b> is connected to an ECU (Electronic Control Unit) <b>50</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) mounted on the vehicle. The main clutch section <b>39</b> of the coupling device <b>30</b> and the ECU <b>50</b> as a control section constitute a drive power transmission control system <b>63</b> in the present invention. The ECU <b>50</b> has connected thereto four speed sensors <b>61</b> arranged in correspondence respectively to the respective wheels <b>13</b>, <b>13</b>, <b>14</b>, <b>14</b>, and these speed sensors <b>61</b> are provided for respectively detecting rotational speeds of the front wheels <b>13</b>, <b>13</b> and the rear wheels <b>14</b>, <b>14</b>. The ECU <b>50</b> is provided therein with a CPU <b>50</b>A, which takes thereinto signals and information from various parts of the vehicle to automatically switch the operation mode of the vehicle into an ordinary mode or an anti-vibration mode. When the front wheels <b>13</b> or the rear wheels <b>14</b> slip at the time of starting in the ordinary mode, the coupling device <b>30</b> is brought into, e.g., the direct connection state, whereby the front wheels <b>13</b> and the rear wheels <b>14</b> are connected to each other by the front and rear wheel torque transmission axles <b>62</b>A and <b>62</b>B. When the operation mode of the vehicle is switched into the anti-vibration mode, on the other hand, the connection force at the main clutch section <b>39</b> of the coupling device <b>30</b> is weakened compared with that in the ordinary mode, that is, is confined to be equal to or less than, e.g., 50 percents of that in the direct connection state.

A ROM <b>50</b>B connected to the CPU <b>50</b>A has stored therein an anti-vibration processing program P<b>1</b> shown in <figref idref="DRAWINGS">FIGS. 4 and 5</figref>. The CPU <b>50</b>A repetitively executes the anti-vibration processing program P<b>1</b> at a regular time interval and is automatically switched based on the processing result of the anti-vibration processing program P<b>1</b> to operate in the ordinary mode or the anti-vibration mode.

More specifically, as shown in <figref idref="DRAWINGS">FIG. 4</figref>, upon execution of the anti-vibration processing program P<b>1</b>, it is judged at step S<b>1</b> whether or not the slip of the rear wheels <b>14</b> continues longer than a predetermined reference time period (e.g., 120 milliseconds). Where the slip of the rear wheels <b>14</b> continues longer than the predetermined reference time period (YES at step S<b>1</b>), switching is made at step S<b>2</b> to the anti-vibration mode, and the CPU <b>50</b>A leaves this anti-vibration processing program P<b>1</b>.

When the rear wheels <b>14</b> are not in the slip state or when the slip state does not continue longer than the predetermined reference time period (NO at step S<b>1</b>), it is judged at step S<b>3</b> whether or not the present mode is the anti-vibration mode. When it is the anti-vibration mode at this time (YES at step S<b>3</b>), judgment is further made at step S<b>4</b> whether or not a grip state of the rear wheels <b>14</b> (i.e., the state of the rear wheels <b>14</b> gripping the road surface) continues longer than the predetermined reference time period. Where the grip state of the rear wheels <b>14</b> continues longer than the predetermined reference time period (YES at step S<b>4</b>), switching is made at step S<b>5</b> to the ordinary mode to leave the anti-vibration processing program P<b>1</b>. Further, when the rear wheels <b>14</b> is not in the grip state or when the grip state does not continue longer than the predetermined reference time period (NO at step S<b>4</b>), the present mode is kept to be the anti-vibration mode before leaving the anti-vibration processing program P<b>1</b>.

Further, when the rear wheels <b>14</b> are not in the slip state or the slip state does not continue longer than the predetermined reference time period (NO at step S<b>1</b>) and when the present mode is not the anti-vibration mode (NO at step S<b>3</b>), the CPU <b>50</b>A leaves this anti-vibration processing program P<b>1</b> with the present mode being kept to be the ordinary mode. That is, the anti-vibration processing program P<b>1</b> in the present embodiment is constructed to execute the mode switching on the condition that either of the slip state and grip state of the rear wheels <b>14</b> continues longer than the predetermined reference time period. Thus, control is performed not to execute the mode switching in a slip state of such a level that does not cause the generation of vibration.

In the foregoing anti-vibration processing program P<b>1</b>, whether the rear wheels <b>14</b> are in the slip state or the grip state is judged in dependence on whether or not the acceleration of the rear wheels <b>14</b> has exceeded a predetermined threshold value. Further, the time period for which each of the slip state and the grip state continues is timed by a counter function incorporated in the anti-vibration processing program P<b>1</b>.

Specifically, as the detailed construction of the anti-vibration processing program P<b>1</b> is shown in <figref idref="DRAWINGS">FIG. 5</figref>, the CPU <b>50</b>A takes thereinto at step S<b>11</b> the rotational speeds of the rear wheels <b>14</b> detected by the speed sensors <b>61</b> upon execution of the anti-vibration processing program P<b>1</b>. Then, at step S<b>12</b>, the CPU <b>50</b>A calculates as the acceleration of the rear wheels <b>14</b> the difference between the rotational speed of the rear wheels <b>14</b> which was taken when the anti-vibration processing program P<b>1</b> was executed at the last time and the rotational speed of the rear wheels <b>14</b> which was taken this time. In this particular embodiment, in calculating the acceleration of the rear wheels <b>14</b>, the difference between the last rotational speed and the present rotational speed is calculated for each of the left and right rear wheels <b>14</b>, <b>14</b>, and a greater one of the two differences so calculated is taken as the acceleration of the rear wheels <b>14</b>. Instead, in calculating the acceleration of the rear wheels <b>14</b>, the rotational speed of the rear wheels <b>14</b> at each of the last time and the present time may be obtained as the average rotational speed of the left and right rear wheels <b>14</b>, <b>14</b>. Then, at step S<b>13</b>, the CPU <b>50</b>A judges whether or not the calculated acceleration is greater than a predetermined threshold value for acceleration judgment. For example, this predetermined threshold value for acceleration judgment is an acceleration which corresponds to 0.7 G (approximately 6.86 m/s²) where converted into the acceleration of the vehicle on the assumption that all of the respective wheels are completely in the grip state.

When the acceleration of the rear wheels <b>14</b> is greater than the predetermined threshold value for acceleration judgment (YES at step S<b>13</b>), a slip counter for timing a slip time period is incremented by “1”, and at the same time, a grip counter for timing a grip time period is reset at step S<b>14</b>. Then, switching to the anti-vibration mode is made at step S<b>2</b> if the value of the slip counter has become greater than a predetermined threshold value for the slip counter (YES at step S<b>15</b>), that is, where the slip time period continues longer than the predetermined time period. However, switching to the ordinary mode is made at step S<b>5</b> if the value of the slip counter is not greater than the predetermined threshold value for the slip counter (NO at step S<b>15</b>), that is, where the slip time period does not continue longer than the predetermined time period.

On the contrary, where the acceleration of the rear wheels <b>14</b> is not greater than the predetermined threshold value for acceleration judgment (NO at step S<b>13</b>), it is judged at step S<b>3</b> whether or not the present mode is the anti-vibration mode. If it is the anti-vibration mode (YES at step S<b>3</b>), the slip counter is reset, and the grip counter is incremented by “1” at step S<b>16</b>. Then, switching to the ordinary mode is made at step S<b>5</b> if the value of the grip counter has become greater than a predetermined threshold value for the grip counter (YES at step S<b>17</b>), that is, where the grip time period continues longer than the predetermined time period. However, switching to the anti-vibration mode is made at step S<b>2</b> if the value of the grip counter is not greater than the predetermined threshold value for the grip counter (NO at step S<b>17</b>), that is, where the grip time period does not continue longer than the predetermined time period.

Further, where the acceleration of the rear wheels <b>14</b> is not greater than the acceleration threshold value (NO at step S<b>13</b>), where the present mode is not the anti-vibration mode (NO at step S<b>3</b>) and where the value of the slip counter is not zero (YES at step S<b>18</b>), the slip counter is reset at step S<b>19</b> before the operation mode is then set to the ordinary mode at step S<b>5</b>. Where the step counter is zero at step S<b>18</b> (NO at step S<b>18</b>), on the contrary, the ordinary mode is kept as it is at step S<b>5</b> without resetting the slip counter at step S<b>19</b>.

Next, the function and advantages of the anti-vibration processing program P<b>1</b> will be described along with the operation of the vehicle.

When an ignition switch (not shown) provided on the vehicle is turn to “ON”, the engine <b>10</b> is started. When the driver shifts a shift lever by the driver's seat to a traveling range like a drive range, a torque is transmitted from the transmission <b>18</b> to the propeller shaft <b>12</b>. In this state, no drive electric current is flown through the electromagnetic coil <b>36</b> of the coupling device <b>30</b>, so that the main inner and outer clutch plates of the main clutch section <b>39</b> remain separated from each other, that is, remain in a two-wheel drive state.

By the way, the front wheels <b>13</b> and the rear wheels <b>14</b> are not necessarily the same in their states of gripping the road surface. That is, since in this particular embodiment, the engine <b>10</b> which is a heavy component is arranged at the front side of the vehicle, the front wheels <b>13</b> with a relatively heavy load applied thereon is less liable to slip than the rear wheels <b>14</b>. For this reason, in this particular embodiment, the front wheels <b>13</b> serve as prime drive wheels. However, it may be the case that either or both of the left and right front wheels <b>13</b>, <b>13</b> slip when the vehicle travels on a low friction road such as a snowy road or the like or when an extraordinarily large torque is applied to the front wheels <b>13</b>, <b>13</b>. If one of the front wheels <b>13</b> slipped, the differential function of the front differential <b>11</b> would cause the torque not to be transmitted to the other front wheel <b>13</b>, whereby the vehicle would be unable to start. In this event, an attempt may be made to heighten the rate of torque distribution to the rear wheels <b>14</b>, <b>14</b> and to end the slip of the one front wheel <b>13</b> by strengthening the connection force of the main clutch section <b>39</b> so that the vehicle can start steadily with the traction being distributed to the four wheels. This attempt would result in ending the slip to make the vehicle start or would result in causing the rear wheels <b>14</b> to slip as well. If the rear wheels <b>14</b> also slipped, vibration would be generated on the torque transmission axles <b>62</b>A, <b>62</b>B on the front and rear wheel sides. That is, with the front wheels <b>13</b> slipping and the rear wheels <b>14</b> not slipping, almost all of the torque from the engine <b>10</b> is transmitted to the rear wheels <b>14</b> side, during which torsion is generated and accumulated on the propeller shaft <b>12</b>. At the moment when the rear wheels <b>14</b> begin to slip in this state, the torsion which has been generated and accumulated on the propeller shaft <b>12</b> is released instantaneously. For this reason and because the rear wheels <b>14</b>, <b>14</b> and the components rotatable bodily therewith are also large in inertia, it is liable that the front and rear wheel torque transmission axles <b>62</b>A, <b>62</b>B are caused to vibrate.

With the construction in the present embodiment, however, where the slip state of the front wheels <b>13</b> continues longer than the predetermined reference time period, the operation mode is automatically switched to the anti-vibration mode. As a consequence, the connection force of the main clutch section <b>39</b> of the coupling device <b>30</b> is weakened compared with that in the direct connection state, and an intermediate portion between the front and rear wheel torque transmission axles <b>62</b>A, <b>62</b>B is substantially divided, so that the resilient force caused by the torsion of the torque transmission axles <b>62</b>A, <b>62</b>B can be released at the intermediate portion. Although some vibration is produced on the torque transmission axles <b>62</b>A, <b>62</b>B even at this moment, the vibration is quite small and is damped quickly because the front wheel torque transmission axle <b>62</b>A is separated from the torque transmission axle <b>62</b>B connected to the rear wheels <b>14</b> and the like which have a large inertia during the slip. Accordingly, it does not occur that the driver is given an unpleasant feeling or an anxious feeling.

Thereafter, where the grip state continues longer than the predetermined reference time period after the rear wheels <b>14</b> are brought into the grip state, the operation mode is restored from the anti-vibration mode to the ordinary mode.

As described above, in the drive system control method and the drive power transmission control system <b>63</b> for the vehicle, the intermediate portion between the front and rear wheel torque transmission axles <b>62</b>A, <b>62</b>B is substantially divided when the rear wheels <b>14</b> slip at the time of the vehicle starting. Thus, though the resilient force is produced by the torsion of the torque transmission axles <b>62</b>A, <b>62</b>B which results from the slip of either of the front wheels <b>13</b> and the rear wheels <b>14</b>, such resilient force is released at the intermediate portion between the torque transmission axles <b>62</b>A, <b>62</b>B, so that the vibration the vehicle suffers at the starting can be suppressed to be smaller than that in the prior art.

Second Embodiment

A second embodiment is different from the foregoing first embodiment in the construction for an anti-vibration processing executed by the drive power transmission control system <b>63</b>. Hereinafter, description will be made regarding the constructions which are different from those in the first embodiment, and repetitive description on the same components as those in the first embodiment will be omitted by putting the same reference numerals on the same components.

The CPU <b>50</b>A of the ECU <b>50</b> provided in the drive power transmission control system <b>63</b> in this second embodiment executes a phenomenon-dependent anti-vibration processing program P<b>2</b> including the anti-vibration processing program P<b>1</b> described in the first embodiment, at a regular time interval. As shown in <figref idref="DRAWINGS">FIG. 6</figref>, when the phenomenon-dependent anti-vibration processing program P<b>2</b> is executed, judgment is made at step S<b>20</b> as to whether or not the state that the front wheels <b>13</b> only are slipping continues longer than a predetermined reference time period.

In this second embodiment, whether the front wheels <b>13</b> are in the slip state or the grip state is judged in dependence on whether or not the acceleration of the front wheels <b>13</b> has exceeded a predetermined threshold value. Further, the time period for which each of the slip state and the grip state continues is timed by a counter function incorporated for this step S<b>20</b> though the details thereof are substantially the same as those shown at step S<b>1</b> in <figref idref="DRAWINGS">FIG. 5</figref> and therefore are not shown in <figref idref="DRAWINGS">FIG. 6</figref> for the sake of brevity. Further, in calculating the acceleration of the front wheels <b>13</b>, the difference between the last rotational speed and the present rotational speed may be calculated for each of the left and right front wheels <b>13</b>, and a greater one of the two differences so calculated is taken as the acceleration of the front wheels <b>13</b>. Instead, in calculating the acceleration of the front wheels <b>13</b>, the rotational speed of the front wheels <b>13</b> at each of the last time and the present time may be obtained as the average rotational speed of the left and right front wheels <b>13</b>.

Where the state that the front wheels <b>13</b> only are slipping continues longer than the predetermined reference time period (YES at step S<b>20</b>), the routine proceeds to another subsequent step S<b>22</b> following the next step S<b>21</b> after storing “1” in a flag F<b>1</b> at the next step S<b>21</b>, and where it is not the case (NO at step S<b>20</b>), the routine proceeds to the another subsequent step S<b>22</b> without altering the content of the flag F<b>1</b>. At the another subsequent step S<b>22</b>, judgment is made of whether or not the flag F<b>1</b> indicates “1”, and if it indicates “1” (YES at step S<b>22</b>), the anti-vibration processing program P<b>1</b> described in the foregoing first embodiment is executed.

Subsequently, following the leaving of the anti-vibration processing program P<b>1</b>, judgment is made at step S<b>23</b> of whether or not the switching of the operation mode has been executed twice after the flag F<b>1</b> is set to “1”. That is, it is judged whether or not the operation mode is returned back to the ordinary mode after once switched from the ordinary mode to the anti-vibration mode. Where the operation mode is returned to the ordinary mode after once switched to the anti-vibration mode (YES at step S<b>23</b>), the CPU <b>50</b>A resets the flag F<b>1</b> to “0” at step S<b>24</b> and leaves the phenomenon-dependent anti-vibration processing program P<b>2</b>. Where the judgment at step S<b>23</b> is not affirmative (NO at step S<b>23</b>), the CPU <b>50</b>A leaves the phenomenon-dependent anti-vibration processing program P<b>2</b> without resetting the flag F<b>1</b> to “0”.

With the construction in the aforementioned second embodiment, where the front wheels <b>13</b> and the rear wheels <b>14</b> slip simultaneously at the time of vehicle starting, the anti-vibration processing program P<b>1</b> is not executed because such state is different from the state in which the front wheels <b>13</b> only slip (NO at step S<b>20</b>). Similarly, where the rear wheels <b>14</b> only slip at the time of vehicle starting, the anti-vibration processing program P<b>1</b> is not executed because such state is also different from the state in which the front wheels <b>13</b> only slip (NO at step S<b>20</b>). On the contrary, where the front wheels <b>13</b> only continue to slip longer than the predetermined reference time period (YES at step S<b>20</b>), the anti-vibration processing program P<b>1</b> is executed. That is, only in the case that the rear wheels <b>14</b> slip following the slip of the front wheels <b>13</b>, the operation mode is switched from the ordinary mode to the anti-vibration mode. As a consequence, even when the four wheels slip simultaneously in the ordinary mode (i.e., four-wheel drive mode), it does not occur that the operation mode is switched to the anti-vibration mode, so that the vehicle can start steadily on a slippery road surface.

Third Embodiment

In this third embodiment, the present invention is applied to a vehicle of the center differential type shown in <figref idref="DRAWINGS">FIG. 7</figref>. The vehicle is provided with a center differential <b>71</b> at the mid position in the front-rear direction. An input portion of the center differential <b>71</b> is connected to the engine <b>10</b>. The center differential <b>71</b> is provided with a pair of output portions, one of which is connected to front side propeller shaft <b>12</b>A extending from the front differential <b>11</b> and the other of which is connected to a rear side propeller shaft <b>12</b>B extending from the rear differential <b>17</b>. Thus, the front wheels <b>13</b> and the rear wheels <b>14</b> can be driven by the engine <b>10</b> with the relative rotation being permitted therebetween.

In the present embodiment, the front wheel torque transmission axle <b>62</b>A is composed of the front side propeller shaft <b>12</b>A, the front differential <b>11</b> and the constant velocity joints <b>13</b>A provided between the front differential <b>11</b> and the front wheels <b>13</b>, whereas the rear wheel torque transmission axle <b>62</b>B is composed of the rear side propeller shaft <b>12</b>B, the rear differential <b>17</b> and the constant velocity joints <b>14</b>A provided between the rear differential <b>17</b> and the rear wheels <b>14</b>.

Between the front and rear side propeller shafts <b>12</b>A, <b>12</b>B, a clutch device <b>72</b> featuring the present invention is provided in parallel relation with the center differential <b>71</b>. More specifically, a front side shaft <b>73</b> connected to a rear end of the front side propeller shaft <b>12</b>A through gears (not numbered) is arranged in axial alignment with a rear side shaft <b>74</b> connected to a front end of the rear side propeller shaft <b>12</b>B through gears (not numbered), and plural clutch plates (e.g., inner or outer clutch plates) provided on the front side shaft <b>73</b> are frictionally engageable with plural clutch plates (e.g., outer or inner clutch plates) provided on the rear side shaft <b>74</b>. Thus, the connection force between the front wheel torque transmission shaft <b>62</b>A and the rear wheel torque transmission shaft <b>62</b>B is made to be variable by the clutch device <b>72</b> in dependence on the driving or traveling state of the vehicle. Other constructions not described above are the same as those corresponding thereto in the foregoing first embodiment, and therefore, description on such other constructions are omitted for the sake of brevity.

The same functions and advantages as those in the foregoing first embodiment can be accomplished even in the aforementioned vehicle of the center differential type to which the present invention is applied.

Further Embodiments or Modifications

The present invention is not limited to the foregoing embodiments. For example, further embodiments or modifications which will be described hereafter are also encompassed in the scope of the present invention. Moreover, the present invention can be practiced in various forms which are modified or altered not to depart from the gist of the present invention, in addition to the further embodiments or modifications described hereafter.

(1) The foregoing first embodiment takes the construction that the intermediate portion between the torque transmission axles <b>62</b>A, <b>62</b>B is substantially divided when the operation mode is switched to the anti-vibration mode. Instead, in the anti-vibration mode, the coupling device <b>30</b> may be kept in the state of a so-called “half clutch connection” so that the resilient force produced by the torsion which is accumulated on the torque transmission axles <b>62</b>A, <b>62</b>B may be gradually released to suppress the vibration.

(2) Although the vehicle in the foregoing first embodiment is of the type that the front wheels <b>13</b> act as prime drive wheels and that the rear wheels <b>14</b> act as secondary or sub-drive wheels, a modification may be made so that the rear wheels <b>14</b> act as prime drive wheels and that the front wheels <b>13</b> act as secondary or sub-drive wheels. In this modification, the anti-vibration processing program P<b>1</b> shown in <figref idref="DRAWINGS">FIG. 5</figref> is modified or altered to calculate the acceleration of the front wheels <b>13</b> in place of the acceleration of the rear wheels <b>14</b> and to judge whether or not the acceleration of the front wheels <b>13</b> continues to exceed a predetermined threshold value for a longer time period than a predetermined reference time period in the same manner as described with respect to the rear wheels <b>14</b> with reference to <figref idref="DRAWINGS">FIG. 5</figref>.

(3) The foregoing first embodiment takes the construction that the operation mode is switched from the ordinary mode to the anti-vibration mode where the rear wheels <b>14</b> continue to slip longer than the predetermined reference time period. Instead, there may be taken another construction wherein the rotational amount of the prime drive wheels in the slip state is detected so that the operation mode can be switched from the ordinary mode to the anti-vibration mode when the detected rotational amount exceeds a predetermined reference value.

(4) It is not judged in any of the foregoing first and second embodiments whether or not the vehicle is beginning to start. Another modified embodiment may take another construction wherein the vehicle is assumed to be at the time of starting while it travels at a lower speed than a predetermined speed (e.g., 15 km/h) and wherein the operation mode is automatically switched from the ordinary mode to the anti-vibration mode only at the time of such starting, but is not automatically switched from the ordinary mode to the anti-vibration mode even at the occurrence of the slip while the vehicle travels at a higher speed than the predetermined speed. With this construction taken, the operation mode is prevented from being automatically switched to the anti-vibration mode during the traveling at any of medium and high speeds, so that the stable traveling of the vehicle can be realized.

(5) The foregoing second embodiment takes the construction that the switching of the operation mode is effected on the condition that the rear wheels <b>14</b> slip following the slip of the front wheels <b>13</b>. However, still another modification may take the construction that the switching of the operation mode is executed only where the rear wheels <b>14</b> begin to slip within a predetermined time period subsequent to the slip of the front wheels <b>13</b>, but is not executed where the rear wheels <b>14</b> begin to slip after the expiration of the predetermined time period.

Various features and many of the attendant advantages in the foregoing embodiments will be summarized as follows:

In the drive system control method and the drive power transmission control system <b>63</b> in one of the foregoing embodiments, the resilient force produced by torsion of the torque transmission axles <b>62</b>A, <b>62</b>B which is caused by the slip of the prime drive wheels <b>13</b> or the secondary drive wheels <b>14</b> is released between the torque transmission axles <b>62</b>A, <b>62</b>B, so that the vibration which would otherwise be generated when the vehicle performs the four-wheel drive starting can be suppressed compared with that in the prior art.

In the drive system control method and the drive power transmission control system <b>63</b> in one of the foregoing embodiments, on the condition that the vehicle is beginning to travel and that the slip of one paired wheels of the front wheels <b>13</b> and the rear wheels <b>14</b> occurs following the occurrence of the slip of the other paired wheels, the operation mode is switched from the ordinary mode to the anti-vibration mode to confine the connection force of the clutch device <b>30</b> to be equal to or less than a predetermined connection force which is able to suppress the vibration of the torque transmission axles <b>62</b>A, <b>62</b>B. Thus, since the switching from the ordinary mode to the anti-vibration mode is restricted to be done at the time of the vehicle starting, the operation mode is prevented from being automatically switched to the anti-vibration mode during the traveling at any of medium and high speeds, so that the stable traveling of the vehicle can be realized. Further, since the switching of the operation mode is restricted to be effected where the slip of one paired wheels of the front wheels <b>13</b> and the rear wheels <b>14</b> occurs following the occurrence of the slip of the other paired wheels, the operation mode is prevented from being switched to the anti-vibration mode where the both paired wheels of the front wheels <b>13</b> and the rear wheels <b>14</b> slip simultaneously, so that it can be realized to make the vehicle start steadily even on a slippery road.

In the drive system control method and the drive power transmission control system <b>63</b> in one of the foregoing embodiments, the operation mode is returned from the anti-vibration mode to the ordinary mode when all of the front and rear wheels <b>13</b>, <b>14</b> come out of the slip state. Thus, it can be realized to effect the switching from the anti-vibration mode to the ordinary mode at such an appropriate time that an increase in the connection force of the clutch device <b>30</b> hardly causes the front and rear wheel torque transmission axles <b>62</b>A, <b>62</b>B to vibrate.

In the drive system control method and the drive power transmission control system <b>63</b> in one of the foregoing embodiments, either one paired wheels of the front wheels <b>13</b> and the rear wheels <b>14</b> are judged to be slipping if the rotational acceleration of either one of the front wheels <b>13</b> or the rear wheels <b>14</b> exceeds a predetermined threshold value. Thus, the occurrence of the slip state can be judged accurately.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.